The present invention relates to a high-resolution array of light-sensing or light-emitting diodes, or a high-resolution array of diodes used for both sensing and emitting light, and to fabrication methods thereof.
Light-emitting and/or light-sensing diode arrays are used for scanning and printing documents in devices such as electrophotographic printers and copiers, and in scanners. These arrays generally comprise a plurality of chips, each containing part of the array. The diodes are formed by diffusion of an impurity into the chip substrate, through windows in a diffusion mask, e.g. by diffusion of zinc (Zn) into a layer of gallium-arsenide-phosphide (GaAsP). Conventional arrays have had resolutions in the range from three hundred to six hundred diodes per inch, often referred to as dots per inch (DPI).
High-quality printing requires light-emitting-diode (LED) arrays with still higher resolutions, however. At least one thousand two hundred dots per inch (1200 DPI) is desirable, but certain problems are encountered in the fabrication of LED arrays with resolutions this high.
There is, for example, the problem of unwanted lateral diffusion. Experience has shown that lateral diffusion up to 1.5 times the diffusion depth must be allowed for. With the conventional diffusion depth of five micrometers (5 .mu.m), zinc can be expected to diffuse up to 7.5 .mu.m sideways from the edges of the diffusion windows. Experiments performed by the inventors indicate that well-shaped diffusion windows need to be at least 5 .mu.m wide, and an undiffused area at least about 5 .mu.m wide is desirable for reliable separation of adjacent diodes. Thus with conventional methods, the minimum diode spacing pitch is 25 .mu.m (5+5+7.5+7.5), but the pitch required for 1200 DPI is 21.2 .mu.m. If the diffusion depth is reduced to enable the spacing to be reduced, with conventional fabrication methods, there is a sharp reduction in emitted light intensity.
Another problem is that of providing adequate electrical contact between the diffused diodes and the electrodes that selectively feed current to the diodes. As the contact area decreases, the contact resistance increases; beyond a certain resistance value, the circuit driving the array (typically an integrated circuit on a separate semiconductor chip, operating with a fixed supply voltage) becomes unable to supply the current necessary for correct emission of light. Keeping the contact resistance within the necessary limit becomes a difficult problem at 1200 DPI, due both to the small size of the diode diffusions and the difficulty of accurate mask alignment.
Yet another problem arises when the LED array chips are fabricated on a wafer that is diced along lines formed by removing part of the diffusion mask. When the impurity is diffused through the diffusion windows, it also diffuses through the dicing line marks, creating unwanted diffusion regions in the vicinity of the dicing lines. At the ends of each chip, these unwanted diffusion regions are located in close proximity to diffusion regions forming diodes in the array, and become a source of light leakage at these diodes. In the worst case, the unwanted diffusions merge with the diode diffusions, causing the chip to be rejected as defective. This problem becomes particularly serious in high-resolution arrays.
Conventional methods thus require considerable improvement if a satisfactory 1200-DPI LED array is to be produced. The same is true of 1200-DPI arrays of light-sensing diodes, or of 1200-DPI arrays of diodes used for both emitting and sensing light. The term light-sensing/emitting diode array will be used generically below to denote an array of diodes that sense, emit, or both sense and emit light.